error detection

[rola248]

I'd like to ask,if i have some equations in a text file such as:
1) 2+45-43
2) 28+50-3
3) 12++36
i need a function that detects the error (eq. 3) when reading the file.is there any possible way to do this in c????

[donbock]

Certainly this kind of syntax checking can be done in C. However, you need to carefully and completely specify the grammer.

For example, the unary minus (negation) can cause a lot of confusion. Do you want these expressions to be legal?
12+-36 (12 plus -36)
4*-8 (4 times -8)

Grammar checking can involve pretty complicated C code. You might want to look at tools made specifically for that job (such as lex or flex) or a language that reduces the edit/build/test/fix cycle time (such as perl).

[rola248]

the first expression is not legal,it should be:
12-36
this is what came to my mind,i know it's not efficient because it doesn't take (,) into consideration,i haven't heard of (lex and flex) before,how can i use them for this purpose??

void finderror(FILE *in,char line[])
{
int i;
int max=countequ(line,in);

for(i=0;i<max;i++)
{read(line,in);
if ((line[i]=='+' &&line[i+1]=='+') || (line[i]=='+' &&line[i+1]=='-') || (line[i]=='+' &&line[i+1]=='*') || (line[i]=='+' &&line[i+1]=='/') || (line[i]=='*' &&line[i+1]=='*') |(line[i]=='*' &&line[i+1]=='+') || (line[i]=='*' &&line[i+1]=='-') || (line[i]=='*' &&line[i+1]=='/')|| (line[i]=='-' &&line[i+1]=='-') || (line[i]=='-' &&line[i+1]=='+') || (line[i]=='-' &&line[i+1]=='*') || (line[i]=='-' &&line[i+1]=='/') || (line[i]=='/' &&line[i+1]=='/') || (line[i]=='/' &&line[i+1]=='+') || (line[i]=='/' &&line[i+1]=='-') || (line[i]=='/' &&line[i+1]=='*'))
printf("ERROR IN LINE : %d",i+1);
else
printf("ALL SUCCESSFUL");
}
}

[donbock]

Look here for details on lex and flex. lex was the original program; flex is a free (but non-Gnu) version.